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ICSE Class VIII Chemistry Matter Around Us - TopperLearning

Anything which occupies space or volume, has mass and can be perceived by our senses is called matter.

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ICSE Class VIII Chemistry Matter Around Us - TopperLearning

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  1. CHEMISTRY MATTER AROUND US Mat Matter ter a aro rou un nd d Us Us Matter  Anything which occupies space or volume, has mass and can be perceived by our senses is called matter.  All materials are made up of matter, and matter consists of particles such as atoms, ions or molecules.  The three states of matter—solid, liquid and gaseous—are based on the differences in physical properties such as mass, volume, shape, rigidity, density and arrangement of particles. Kinetic Theory of Matter The kinetic theory of matter states that all matter is composed of particles which  Have intermolecular spaces between them.  Attract each other with a force.  Are in a continuous random motion. Properties of Matter Solid State Liquid State Gaseous State Mass Solids have a definite mass. Liquids have a definite mass. Liquids do not have a fixed shape but have a fixed volume. Liquids take up the shape of the container in which they are poured. They are less rigid. The space between the particles is slightly more as compared to solids but still very less. The particles of liquids can slip and slide over each other. Gases have a definite mass. Gases neither have a definite shape nor have a definite volume. They fill up the container completely. They are not rigid. Volume, Shape, Rigidity Solids have a definite shape. They maintain their shape even when they are subjected to an external force, i.e. they are rigid. On the basis of kinetic theory – Intermolecul ar space The space between the particles is very less. The particles are much farther apart from one another as compared to solids and liquids. They have a very disorderly arrangement of particles compared to solids and liquids. www.topperlearning.com 2

  2. CHEMISTRY MATTER AROUND US On the basis of kinetic theory – Force of attraction The force of attraction between the particles is strong. Thus, particles in a solid are closely packed. The force of attraction between the particles is strong enough to hold the particles together but not strong enough to hold the particles in a fixed position. Thus, particles in a liquid are not as closely packed as in solids. Liquid particles are free to move around in the liquid only. They can slip and slide over each other. The force of attraction between the particles is negligible; hence, particles of a gas move freely in all directions. Gases can thus mix or diffuse into other gases. On the basis of kinetic theory – Movement Solid particles vibrate only about their mean position. Gaseous particles move with high speed in all directions and can exert pressure on the walls of the container. Gases have least density as the number of particles is least and the intermolecular space is maximum. Density Solids have high density. This is because the number of particles in a solid is more and the intermolecular space is minimum. Liquids have less density as compared to solids because the number of particles is less and the intermolecular space is more. Liquids have one upper free surface. Free surface Solids have an infinite free surface. Gases do not have any free surface. Miscibility or Diffusibility Solids do not diffuse with other solid particles. Liquids may diffuse with other liquid particles. Gaseous particles rapidly diffuse with other gaseous particles. Liquids cannot be compressed much. The compressibility of liquids is almost negligible. Compressibi lity Solids cannot be compressed. Gases can be compressed easily. Examples: LPG cylinders used at home and CNG cylinders used in vehicles. __ Some solids may change their shape when an external force is applied, but when that force is removed, they can regain their original shape. This shows that some solids are elastic. Liquids show a property called viscosity. More viscous liquids flow slowly, while less viscous liquids flow easily. www.topperlearning.com 3

  3. CHEMISTRY MATTER AROUND US Change of State (Interconversion of Matter) The phenomenon of change from one state of matter to another and then back to the original state is called the interconversion of states of matter. It is affected by changes in conditions such as 1. Changing the temperature. 2. Increasing or decreasing the pressure. 3. Changing both temperature and pressure. Interconversion of states of matter www.topperlearning.com 4

  4. CHEMISTRY MATTER AROUND US Term Definition Process of change (at particular temperature) Melting (Solid  Liquid) Melting The process of change of state from solid to liquid on absorbing heat at a particular temperature and at one atmospheric pressure is called melting or fusion. The process of conversion of a substance from the liquid state to the gaseous state at any temperature below its boiling point is called evaporation or vaporisation. Evaporation is a surface phenomenon. The process of change from the gaseous state to the liquid state at a particular temperature is called liquefaction. The process of change of matter from the liquid state to the solid state at a particular temperature is called freezing or solidification. A change of state of a substance directly from solid to gas without changing into a liquid state (or vice versa) is called sublimation. The constant temperature at which a solid becomes liquid upon absorbing heat under normal pressure is called the melting point of that solid. The constant temperature at which a liquid becomes gas upon absorbing heat under normal pressure is called the boiling point of that liquid The constant temperature at which a gas becomes liquid under normal pressure is called the liquefaction point of that liquid. The constant temperature at which a liquid changes into a solid by giving out heat energy is called the freezing point of that liquid. Evaporation (Liquid  Gas) Vaporisation Liquefaction (Gas  Liquid) Liquefaction Freezing (Liquid  Solid) Solidification Sublimation (Solid  Gas) Sublimation Melting point Melting point (Solid → Liquid) Boiling point Boiling point (Liquid → Gas) Liquefaction point Liquefaction point (Gas → Liquid) Freezing point Freezing point (Liquid → Solid) www.topperlearning.com 5

  5. CHEMISTRY MATTER AROUND US Interconversion of Matter on the basis of Kinetic Theory Melting On heating solids, the temperature rises.  Particles gain energy.  Intermolecular force of attraction decreases.  Intermolecular space between the molecules of a solid increases.  At the melting point, particles of solid become free from the fixed position and get converted into a liquid. Liquefaction On cooling gases, the temperature falls.  Particles lose energy.  Intermolecular attraction increases.  Intermolecular space between the particles/molecules of gases decreases.  At liquefaction point, particles slow down, come close and get converted into a liquid. Dalton’s Atomic Theory, 1808 In 1808, the English chemist John Dalton gave a systematic idea about the structure of an atom. His ideas are grouped together and known as Dalton’s atomic theory. Postulates of Dalton’s Atomic Theory 1. All matter is made up of very tiny, indivisible and indestructible particles called atoms. 2. Atoms can neither be created nor be destroyed. 3. Atoms of same elements are alike in all respects. 4. Atoms combine in the ratio of small whole numbers to form compounds or molecules. Modern Atomic Theory  An atom comprises three fundamental sub-atomic particles—electron, proton and neutron.  Atoms of the same element may not be alike in all respects.  Isotopes are atoms of the same element differing in properties. Vaporisation On heating liquids, the temperature rises.  Particles gain energy.  Intermolecular force of attraction decreases.  Intermolecular space between the molecules of a liquid increases.  At the boiling point, particles become completely free and convert into a gas/vapour. Solidification On cooling liquids, the temperature falls.  Particles lose energy.  Intermolecular attraction increases.  Intermolecular space between the particles/molecules of gases decreases.  At solidification/freezing point, particles slow down, come close and get converted into a solid. www.topperlearning.com 6

  6. CHEMISTRY MATTER AROUND US Atoms and Molecules Atom  An atom is the basic unit of matter.  An atom may or may not have an independent existence.  An atom is further divisible into protons, neutrons and electrons. Molecules  Atoms of the same element or different elements can join together to form molecules.  A molecule is the smallest particle of a substance which can normally exist on its own.  A molecule retains the physical and chemical properties of the substance. Atoms – Structure Basic Structure of an Atom  An atom consists of three fundamental or sub-atomic particles—proton, neutron and electron.  The central core or the nucleus of every atom consists of protons and neutrons. Electrons revolve around the nucleus in different orbits. Fundamental particles of an atom Atomic Number and Atomic Mass Number Atomic Number (Z) The number of protons in an atom of an element is called the atomic number of the element. It is denoted by the letter Z. Z = p = e (Atomic no.) (No. of protons) (No. of electrons) Mass Number (A) The sum of the number of protons and the number of neutrons present in the nucleus of an atom of an element is called the mass number of that element. It is denoted by the letter A. A = p + e (Mass no.) (No. of protons) (No. of electrons) www.topperlearning.com 7

  7. CHEMISTRY MATTER AROUND US Electronic Configuration Orbits Electrons revolve around the nucleus in an imaginary path called an orbit or shell.  The maximum number of electrons which can be present in any shell of an atom is given or represented by the formula 2n2.  Here, n is the principal quantum number which is equal to the number of shells as counted from the nucleus. Formula 2n2 2 × (1)2 2 × (2)2 2 × (3)2 2 × (4)2 Shell Shell number (n) Maximum number of electrons in each shell designation K-shell 1 2 L-shell 2 8 M-shell 3 18 N-shell 4 32  The outermost shell cannot hold more than 8 electrons.  As soon as the maximum capacity of a shell is satisfied, a new shell is filled. Electronic Configuration of Sodium Atomic number of sodium = 11. Thus, we know that an atom of sodium contains 11 electrons. The orbit-wise distribution of electrons in a sodium atom is as follows: Orbit number of electrons 2n2 = 2 × 12 = 2 electrons L-shell 2n2 = 2 × 22 = 8 electrons M-shell Remaining = 1 electron Thus, the electronic configuration of sodium is (2,8,1). Relative Atomic Mass The relative atomic mass of an element is defined as the ratio of the average mass of an atom of the element to 1/12th of the mass of one carbon-12 atom. Atomic mass or relative atomic mass is simply the number of times one atom of an element is heavier than either the mass of an atom of hydrogen or the 1/12th mass of an atom of carbon-12. Relative Molecular Mass The relative molecular mass of an element or compound is the ratio of mass of one molecule of a substance to the mass of 1/12th the mass of one atom of carbon-12.  Molecular mass or relative atomic mass is simply the number of times one molecule of an element is heavier than either the mass of an atom of hydrogen or the 1/12th mass of an atom of carbon-12. Isotopes and their application  Atoms of the same elements differing in the number of neutrons in their nuclei are known as isotopes. Thus, isotopes of an element have the same atomic number but different atomic mass number.  Isotopes of an element have similar chemical properties but different physical properties. Maximum number of electrons K-shell www.topperlearning.com 8

  8. CHEMISTRY MATTER AROUND US Formation of Compounds from Atoms 1. Stable and unstable Electronic Configuration Stable electronic configuration Noble gases have stable electronic configuration. Their valance or outermost shell is completely filled. They have 2 electrons (He) or 8 electrons (Ne, Ar, Kr, Xe, Rn) in their outer shell. They do not gain, lose or share electrons. Unstable electronic configuration Atoms of other elements have unstable electronic configuration. Their valance or outermost shell is incompletely filled. They tend to attain stable electronic configuration of the nearest inert gas by  Gaining or losing electrons (electron transfer).  Sharing electrons. 2. Atoms Combine to Form Compounds Electron transfer – Electrovalency Sodium (Na) atom  Electronic configuration = 2,8,1  Nearest inert gas = Neon [2,8]  It loses one electron from the outermost shell (valence shell) to attain stability. Sharing of electrons – Covalency Oxygen (O) atom  Electronic configuration = 2,6  Nearest inert gas = Neon [2,8]  Needs two electrons to attain stability. Chlorine (Cl) atom  Electronic configuration = 2,8,7  Nearest inert gas = Argon [2,8,8]  It gains one electron in the outermost shell (valence shell) to attain stability. Oxygen (O) atom  Electronic configuration = 2,6  Nearest inert gas = Neon [2,8]  Needs two electrons to attain stability. www.topperlearning.com 9

  9. CHEMISTRY MATTER AROUND US Classification of Matter (Elements, Compounds and Mixtures) Matter may be broadly classified into elements, compounds and mixtures. Element  An element is a pure substance composed of only one kind of atom.  An element cannot be broken down into two or more simple substances by any physical or chemical means. Characteristics of an Element 1. An element is made up of only one kind of atom. 2. An element is a pure and homogeneous substance. 3. An element has fixed melting and boiling points. 4. An atom is the smallest particle of an element which takes part in a chemical reaction. 5. An element may chemically react with another element or compound. 6. An element can occur in the solid, liquid or gaseous state. Classification of Elements  Have metallic lustre.  Are good conductors of heat and electricity.  Are malleable and ductile.  Are solids.  Contain one kind of atom (monoatomic).  Do not have lustre.  Are bad conductors of heat and electricity.  Are neither malleable nor ductile.  Are solids, liquids and gases.  Contain two kinds of atoms (monoatomic or diatomic).  Properties are mid-way between metals and non- metals.  Contain one kind of atom (monoatomic).  Are gaseous in nature.  Are chemically inert.  Occur in free state in traces in the atmosphere.  Contain one kind of atom (monoatomic). www.topperlearning.com 10

  10. CHEMISTRY MATTER AROUND US Compound  A compound is a pure substance composed of two or more elements combined chemically in a fixed proportion by mass. Characteristics of Compounds 1. Components in a compound are present in a definite proportion. 2. They have a homogeneous composition. 3. Particles in a compound are of one kind. 4. A compound is made up of one or more atoms of the same or different elements. 5. In a compound, the elements are present in a fixed ratio by mass. 6. A compound can be divided into simpler substances by a chemical process. 7. The physical and chemical properties of a compound are completely different from those of its constituents. A Comparative Study between Elements and Compounds Element 1. It is a pure substance which cannot be converted into simpler substances by any physical or chemical means. 2. It is made up of atoms of only one kind. 3. The molecules are made up of one or more atoms. 4. Elements cannot be broken down into two or more simpler substances by any physical or chemical means. 5. Elements have their own set of properties. Compound 1. It is a pure substance made up of two or more elements combined chemically in a fixed ratio. 2. It is made up of two or more different kinds of atoms. 3. The molecules are made up of two or more atoms. 4. A compound can be divided into simpler substances only by chemical means. 5. Properties of compounds are different from their constituent elements. www.topperlearning.com 11

  11. CHEMISTRY MATTER AROUND US Mixtures  A mixture is defined as matter composed of two or more substances whose particles are in contact but are not chemically combined and have not lost their individual properties.  The properties of a mixture vary according to the proportions of the components present in it. Types of Mixtures Homogeneous mixtures: A mixture which has uniform composition and properties throughout its mass is called a homogeneous mixture. Examples: Sugar solution, salt solution Heterogeneous mixture: A mixture which has different composition and properties in different parts of its mass is called a heterogeneous mixture. Examples: Sand mixed with salt, sugar in oil Types of mixture Homogeneous Solid in solid Alloys (Bronze - Cu, Zn, Sn) Heterogeneous Gun powder (charcoal, sulphur, nitre) Sugar in oil, sand in water Solid in liquid Iodine in alcohol, sugar in carbon disulphide, sugar in water, salt in water Amalgam (Hg + Au) Methanol in water, acetone in water HCl in water Moisture in air Pure air Liquid in solid Liquid in liquid Water in sponge Oil in water, kerosene in water Helium in water Mist, fog Air in industries Gas in liquid Liquid in gas Gas in gas A Comparative Study between Compounds and Mixtures Compound 1. It is obtained by the chemical combination of more than one element. Mixture 1. It is obtained by the physical combination of either elements, compounds or both. 2. The composition of elements present in a compound is fixed. 2. The composition of elements present in a mixture is not fixed. 3. The properties of a compound are different from those of its elements. 3. It shows the properties of all its constituent elements. 4. Its constituents can be separated by using only chemical and electrochemical methods. 4. Its constituents can be separated using physical methods. 5. A compound is always homogeneous in nature. 5. The mixtures can be homogeneous or heterogeneous. www.topperlearning.com 12

  12. CHEMISTRY MATTER AROUND US Methods of Separation Separation of Solid–Solid Mixtures Sublimation To separate the mixture of sublimable solid X + non-sublimable solid Y. Magnetic separation To separate the mixture of magnetic solid X + non-magnetic solid Y. Solvent extraction method To separate the mixture of insoluble solid X + soluble solid Y. Chromatography – For Complex Mixtures To separate the mixture of different solid constituents in a liquid constituent. Separation of Solid–Liquid Mixtures Filtration To separate the mixture of insoluble solid X from liquid component Y. Sedimentation and Decantation Insoluble solid X from liquid component Y. Evaporation To separate the mixture of soluble solid X from liquid component Y. Distillation To separate the mixture of soluble solid X from liquid component Y. Separation of Liquid–Liquid Mixtures Separating funnel To separate the mixture of immiscible heavier liquid X from immiscible lighter liquid Y. Fractional distillation To separate the mixture of miscible liquid X with lower boiling point and miscible liquid Y with higher boiling point. www.topperlearning.com 13

  13. CHEMISTRY MATTER AROUND US Basic Chemistry – Terminology and Reactions Symbol A symbol is a short form of the name of an element. The specific abbreviation used to denote the name of an element is called its symbol. Representation of a Symbol Dalton’s Symbol, 1808  Dalton was the first scientist to use figurative symbols for atoms of some of the elements. His symbols represented the ‘element’ as well as ‘one atom’ of that element. Drawbacks of Dalton’s Symbols  Dalton's symbols for elements were difficult to draw and inconvenient to use. Thus, they are not used any more. Berzelius Symbol, 1814 In 1814, the Swedish Chemist Jöns Jakob Berzelius devised a system using letters of the alphabet. He put forward certain points for presentation. 1. In most cases, the first letter of the name of an element was taken as the symbol for that element and written in capitals. Name Carbon 2. In some cases, the initial letter of the name in capital and its second letter in small were used. Name Calcium 3. The symbols for some elements were derived from their Latin names. English name of the element element Symbol C Symbol Ca Latin name of the Symbol Natrium Sodium Na Kalium Potassium K Argentum Silver Ag  The method suggested by Berzelius forms the basis of the IUPAC (International Union of Pure and Applied Chemistry) system of chemical symbols and formulae. www.topperlearning.com 14

  14. CHEMISTRY MATTER AROUND US Radicals and Valency What are Ions or Radicals? Two or more non-metals which collectively accept or donate one or more electrons and become negatively or positively charged in the process are called radicals. Types of Ions or Radicals Radicals are either positively charged or negatively charged. Positively charged radicals (ions) are called cations, and negatively charged radicals (ions) are called anions. Valency It is the number of hydrogen atoms which can combine with or displace one atom of the element or radical. Number of atoms of Hydrogen (H) element Combining Molecule Valency 1 1 atom of Chlorine (Cl) Hydrogen chloride (HCl) 1 2 1 atom of Oxygen (O) Water (H2O) 2 Variable Valency Sometimes, the same element may exhibit one valency in one compound and another valency in other compound. This property is called variable valency. Example Element Symbol Valencies exhibited (variable valencies) Cu1+, Cu2+ Copper Cu 1, 2 Valency Chart List of common electrovalent positive ions or radicals 1. Monovalent electropositive ions Cuprous Cu+ Copper [I] 2. Bivalent electropositive ions Cupric Cu 3. Trivalent electropositive ions Aluminium 4. Tetra-positive ions Plumbic Pb List of common electrovalent negative ions or radicals 1. Monovalent electronegative ions Acetate CH3COO− 2. Bivalent electronegative ions Sulphate SO4 3. Trivalent electronegative ions Phosphide 4. Tetravalent electronegative ions Carbide C 2– 2+ Copper [II] – 3+ Al 4– 4+ www.topperlearning.com 15

  15. CHEMISTRY MATTER AROUND US Molecular Formula A molecular formula, also known as a chemical formula, is a combination of elemental symbols and subscript numbers which is used to show the composition of a compound. Writing the Molecular Formula Step 1: Write the symbol of a basic radical (element with positive valency) to the LHS and that of the acid radical (element with negative valency) to the RHS. Step 2: Write the valency number of each of the respective radicals at the right hand top of its symbol. Step 3: Interchange the valency number. Ignore the (+) and () signs. Step 4: Write the interchanged number. Step 5: Write the compound’s formula. Chemical Equation The representation of a chemical reaction with the help of chemical formulae of the reactants and products is a chemical equation. The reaction can be represented by either a word equation or a chemical equation using symbols and formulae. Word equation: Chemical equation: How are Chemical Equations Represented?  In a chemical reaction, the reactants are written on the LHS and the products on the RHS of the equation. An arrow (→) pointing towards the products is inserted between the reactants and the products. It also represents the direction of the reaction. A single arrow (→) indicates the direction in which the reaction proceeds.  A double arrow ( ) indicates a reversible reaction, i.e. products recombine to form reactants.  A plus sign (+) is inserted between two or more reactants and products formed.  A chemical reaction can be characterised by factors such as change of state, change in colour, evolution of a gas, change in temperature, formation of a precipitate and evolution of heat, light or sound. Balancing an Equation  In a balanced chemical equation, the total number of atoms of each element in the reactants on the LHS of the equation is the same as the number of atoms in the products formed on the RHS of the equation.  The total mass of the reactants is equal to the total mass of the products, or the number of atoms of each element before the reaction and after the reaction is equal. www.topperlearning.com 16

  16. CHEMISTRY MATTER AROUND US Steps involved in Balancing a Chemical Equation Consider the chemical reaction between magnesium and oxygen to understand the steps involved in balancing a chemical equation. 1. Let us first write the word equation for this reaction. Magnesium + Oxygen → Magnesium oxide 2. Write the chemical equation for the reaction between magnesium and oxygen. Mg + O2→ MgO 3. Count the number of times an element occurs on both LHS and RHS in this equation. Mg + O2→ MgO Component Reactant Magnesium Oxygen This is an unbalanced equation. 4.  Choose a reactant or product which has the maximum number of atoms in it. In this equation, we shall select MgO, i.e. magnesium oxide and the element oxygen in it.  To balance the oxygen atoms, let us multiply the magnesium oxide molecule by 2 on the RHS. The equation can now be expressed as Mg + O2→ 2MgO Component Reactant Magnesium Oxygen 5. There are two oxygen atoms on either side of the equation, but one magnesium atom on the reactant's side and two on the product's side. Therefore, multiply the magnesium atom by 2 on the LHS. Component Reactant Magnesium Oxygen The balanced equation is 2Mg + O2→ 2MgO  The number of atoms of each element of reactants = The number of atoms of each element of products Product 1 1 1 2 Product 2 2 1 2 Product 2 2 2 2 www.topperlearning.com 17

  17. CHEMISTRY MATTER AROUND US Chemical Reaction Chemical reactions involve the transfer of matter from one substance to another substance during a chemical change. Main Types of Chemical Reactions 1. Direct combination or synthesis 2. Decomposition A chemical reaction in which a single compound splits into two or more simple substances is called a decomposition reaction. 3. Thermal decomposition simpler compounds. The simpler compounds do not reunite to form the original compound on cooling. 4. Reversible reaction called a reversible reaction. 5. Thermal dissociation substances on the application of heat is called a thermal dissociation reaction. It is a reversible reaction. 6. Displacement Reactions in which the more reactive element displaces the less reactive element from its compound are called displacement reactions. 7. Double displacement new compounds are called double displacement reactions. In double displacement reactions, the two reactants taking part are generally water soluble, and one of the products is soluble and the other being insoluble separates out as a solid. 8. Double decomposition to form two new compounds by the mutual exchange of radicals. Usually, a solid is formed as a result of the reaction. These reactions are of two types—precipitation reactions and neutralisation reactions. Precipitation reactions The insoluble solid formed during double displacement reactions is called a precipitate. Reactions in which a precipitate is formed as one of the products are also called precipitation reactions. Neutralisation reactions The reaction between an acid and a base to form a salt and water is called a neutralisation reaction. 9. Catalytic reaction Catalyst: It is a compound which alters the rate of a reaction but does not take part in a chemical reaction. Examples: V2O5,Fe2O3,finely divided iron (Fe), platinum (Pt) A chemical reaction in which two or more substances combine to form a single product is called a combination reaction or synthesis. During thermal decomposition, a chemical compound breaks into A reaction in which the direction of a chemical change can be easily reversed by changing the conditions under which the reaction occurs is A reaction in which a substance dissociates into two or more simpler Reactions in which ions of the reactants exchange places to form two A type of chemical change in which two compounds in a solution react A chemical reaction which involves the use of a catalyst. www.topperlearning.com 18

  18. CHEMISTRY MATTER AROUND US 10. Exothermic and Endothermic reactions Exothermic reaction Chemical reactions which proceed with the evolution of heat energy are called exothermic reactions. A + B → C + Δ Endothermic reactions Chemical reactions which proceed with the absorption of heat energy are called exothermic reactions. Oxidation Reactions Reactions which involve the addition of oxygen or the removal of hydrogen are called oxidation reactions. What is an oxidising agent? The substance which loses oxygen or an electronegative radical is called an oxidising agent. The substance which gains hydrogen or an electropositive radical is also called an oxidising agent. Examples: Oxygen, chlorine Reduction reactions The addition of hydrogen to a substance is called reduction. The removal of oxygen from a substance is also called reduction. Reactions which involve the addition of hydrogen or removal of oxygen are called reduction reactions. What is a reducing agent? The substance which loses hydrogen or an electronegative radical is called a reducing agent. The substance which gains oxygen or an electronegative radical is also called a reducing agent. Examples: Hydrogen, carbon monoxide, hydrogen sulphide Redox reactions Oxidation and reduction reactions occur together. When oxidation occurs in one substance, reduction occurs in the other substance. Such reactions are called redox reactions. When an atom or a group of atoms loses electrons, oxidation occurs. In this reaction, hydrogen sulphide is oxidised to sulphur. 11. Oxidation and reduction reactions (redox) Differences between Oxidation and Reduction Oxidation 1. Addition of oxygen 2. Removal of hydrogen 3. Loss of electrons Reduction 1. Removal of oxygen 2. Addition of hydrogen 3. Gain of electrons Important Links Buy class 10th pack View Past session Watch Video Lesson Syllabus Download Visit us on: http://www.topperlearning.com www.topperlearning.com 19

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